Method for exploitation of hydrocarbons from a sedimentary basin by means of a basin simulation taking account of geomechanical effects

ABSTRACT

Method for exploitation of a sedimentary basin containing hydrocarbons by means of a basin simulation. 
     On the basis of a reconstitution of the formation of the first of the layers of the basin, for at least one of the layers underlying the first layer, a conjoint basin simulation of the layer concerned and at least one underlying layer is carried out. A conjoint geomechanical simulation of the layer concerned and at least one underlying layer is then carried out. If the deviation between some of the parameters from the geomechanical simulation and those from the basin simulation is above a predefined threshold the previous steps are repeated applying a correction to the basin simulation. The basin is then exploited as a function of the results of the basin simulation. 
     Application in particular to the exploration and exploitation of hydrocarbon deposits.

The present invention concerns the exploration and exploitation ofhydrocarbon deposits or geological gas storage sites.

The present invention more particularly concerns basin modeling appliednotably to evaluating the hydrocarbon potential of a sedimentary basinwith a complex geological history.

Hydrocarbon exploration consists in looking for hydrocarbon deposits ina sedimentary basin. The understanding of the principles governing thegenesis of hydrocarbons and their links with the geological history ofthe subsoil have made it possible to develop methods for evaluation ofthe hydrocarbon potential of a sedimentary basin. The general approachto the evaluation of the hydrocarbon potential of a sedimentary basinincludes to-and-fro movements between a prediction of the hydrocarbonpotential of the sedimentary basin, produced on the basis of availableinformation concerning the basin being studied (outcrops, seismiccampaigns, drilling, for example) and exploration drillings in thevarious zones having the greatest potential in order to confirm or torule out the potential previously predicted and to acquire new data tofeed new, more accurate studies.

The exploitation of an hydrocarbon deposit consists in selecting theareas of the deposit having the greatest hydrocarbon potential on thebasis of information collected during the hydrocarbon exploration phase,defining optimum exploitation schemes for those zones (for example usinga reservoir simulation in order to define the number and positions ofthe exploitation wells making possible optimum hydrocarbon recovery),drilling exploitation wells and generally speaking in setting up theproduction infrastructures necessary for the development of the deposit.

A sedimentary basin is the result of the deposition of sediment in adepression in the earth's crust over geological periods of time. Thesesediments, loose and rich in water, will be subjected during theirprogressive burial in the basin to pressure and temperature conditionsthat will transform them into compact sedimentary rocks, termedgeological layers.

The present-day architecture of a sedimentary basin is notably theresult of mechanical deformation of the subsoil over geological timeperiods. This deformation includes at the very least compacting of thegeological layers due to the progressive burying of these layers in thebasin because of the effect of new sediments. However, a sedimentarybasin is also most often subjected to tectonic movements of largeamplitude, generating upthrusts in the geological layers, for example,or even faults creating a rupture in the geological layers.

For its part, the nature of the hydrocarbons present in a sedimentarybasin results in particular from the type of organic material present inthe sediments that have been deposited, and also the pressure andtemperature conditions to which the basin is subjected over geologicaltime periods.

FIG. 1 is a diagrammatic representation of a sedimentary basin includinga plurality of geological layers (a,c) delimited by sedimentaryinterfaces (b) crossed by a fault line (e) and an accumulation (d) ofhydrocarbons in one of the geological layers of the basin (c) concerned.

The formation of a sedimentary basin therefore involves a large numberof complex physical and chemical processes, which are moreover able tointeract. Faced with this complexity, the prediction of the hydrocarbonpotential of a sedimentary basin requires computer tools making itpossible to simulate as realistically as possible the physical andchemical phenomena involved in the formation of the basin being studied.

This type of reconstitution of the history of the formation of asedimentary basin, also termed basin modeling, is most often carried outby means of a family of computer tools making it possible to simulate inone, two or three dimensions the sedimentary, tectonic, thermal,hydrodynamic and organic and inorganic chemistry processes that areoperative during the formation of a basin.

Basin modeling conventionally includes three steps:

-   -   a geo-modeling step that consists in constructing a meshed        representation of the basin being studied. This meshed        representation is most often structured in layers, i.e. a group        of meshes is assigned to each geological layer of the modeled        basin. Each mesh of this meshed representation then has entered        into it one or more petrophysical properties, such as porosity,        facies (clay, sand, etc.) or again the organic material content.        The construction of this model is based on data acquired from        seismic campaigns, measurements in wells, core samples, etc.    -   a step of structured reconstruction of the architecture of the        basin: it is a question of reconstructing the past architectures        of the basin. To this end, the meshed representation constructed        in the preceding step is deformed in order to represent the        anti-chronological evolution of the architecture of the subsoil        during geological time periods, for different time periods also        known as time steps.    -   a basin simulation step: this is a numerical simulation of a        selection of physical and chemical phenomena occurring during        the evolution of the basin and contributing to the formation of        the petroleum traps. This basin simulation is effected time step        by time step and relying for each time step on the meshed        representations constructed for each time step in the preceding        step. In particular, a basin simulation supplies a predictive        mapping of the subsoil indicating the probable location of the        deposits together with the concentration, nature and pressure of        the hydrocarbons trapped therein.

By supplying quantitative and reliable information, this integratedbasin modeling approach makes it possible to increase the success ratewhen drilling an exploration well.

Generally speaking, a sedimentary basin may be subjected over itshistory to mechanical stresses characterized by components in threedimensions in space, these stresses being local or regional and variableover time. These mechanical stresses are on the one hand induced by thesediment deposits themselves. In this case, the mechanical stressesinclude a vertical component linked to the weight of the sediments onthe layers already deposited but most often also include a horizontalcomponent, the sedimentary deposits generally not being invariantlaterally. On the other hand, throughout its formation, a sedimentarybasin is subjected to mechanical stresses induced by tectonic movementslinked to terrestrial geodynamics, such as movements in extension(causing the opening of the basin with the formation of a rift, forexample) or movements in compression (causing thrusts and upthrusts inthe basin, etc.). These tectonic movements most often induce variationsof mechanical stresses in three dimensions in space. Note that a layeralready deposited will be subject to variations of stresses induced bythe tectonic movements to which a sedimentary basin is subjectedthroughout its formation.

Very conventionally, as described for example in the document (SchneiderF., 2003), the basin simulation software assumes only verticalvariations of the mechanical stresses affecting a sedimentary basin. Tobe more precise, the basin simulation software takes into account onlythe vertical component of the variations of mechanical stresses inducedby the weights of the successive sedimentary deposits over time. This isreferred to as 1D simulation of the mechanical effects.

Not taking account of the three-dimensional variations of the mechanicalstresses can have serious consequences in the evaluation of thehydrocarbon potential of a sedimentary basin. In fact, the horizontalmechanical stresses can for example generate fractures or upthrusts inthe geological layers of the basin, which can greatly modify thecharacteristics of the petroleum deposits and the rocks covering thesedeposits and consequently the preferred flow paths, the pressure levelsin the basin, the location of the hydrocarbon traps, etc.

Taking into account the variations in three dimensions of the mechanicalstresses to which a sedimentary basin is subjected during its genesistherefore appears to be important for a realistic prediction of thehydrocarbon potential of said basin, all the more so when that basin hasa complex geological history.

PRIOR ART

The following documents will be cited in the description:

-   Nayroles, B., G. Touzot and P. Villon. 1991. The diffuse    approximation. C. R. Acad. Sci., Paris, series II. 313: 293-296.-   Scheichl, R., Masson, R., Wendebourg, J., Decoupling and Block    Preconditioning for Sedimentary Basin Simulations, Computational    Geosciences 7(4), pp. 295-318, 2003.-   Schneider F., Modelling multi-phase flow of petroleum at the    sedimentary basin scale. Journal of Geochemical exploration    78-79 (2003) 693-696.-   Schneider, F., S. Wolf, I. Faille, D. Pot, A 3D Basin Model for    Hydrocarbon Potential Evaluation: Application to Congo Offshore, Oil    & Gas Science and Technology—Rev. IFP, Vol. 55 (2000), No. 1, pp.    3-13.-   Steckler, M. S., and A. B. Watts, Subsidence of the Atlantic-type    continental margin off New York, Earth Planet, Sci. Lett., 41, 1-13,    1978.-   Zoback, M. D., Reservoir Geomechanics, 2010.-   Zienkiewicz, O. C., R L Taylor and J Z Zhu, The Finite Element    Method: Its Basis and Fundamentals (seventh ed.), 2013.

There is known the U.S. Pat. No. 8,271,243 B2 that disclosesco-operation between a basin simulation and a geomechanical simulationwith a view to taking account of three-dimensional geomechanical effectsin basin modeling.

Geomechanical modeling, as applied to the field of evaluating thehydrocarbon potential of a sedimentary basin, conventionally useshistories of pressure, temperature and saturation, quantities ofsediments deposited or eroded, tectonic stresses where applicable, andlaws of behavior associated with the various lithologies of the domainbeing modeled, in order to describe the geomechanical behavior of abasin over geological time periods.

Geomechanical modeling generally comprises two phases:

-   -   a geo-modeling phase that consists in the construction of a        meshing of the sedimentary basin to be studied. As in basin        simulation, this meshed representation is most often structured        in layers, i.e. a group of meshes is assigned to each geological        layer of the modeled basin. Each mesh of this meshed        representation is then associated with, for example, a        geomechanical behavior law, a porosity, a pressure, a stress or        a density. The construction of this model is based on data        acquired from seismic campaigns, measurements in wells, core        samples, etc. For reasons linked to the numerical solution of        the equations involved in basin simulation and in geomechanical        simulation, a meshed representation suitable for a basin        simulation may be different from a meshed representation        suitable for a geomechanical simulation.    -   a step of numerical geomechanical simulation, which makes it        possible to calculate the evolution over time of the        distribution of stresses in each of the meshes of the meshed        representation together with the resulting deformations. This        type of technique is used in the petroleum industry, but is also        used in the geotechnical field, for example. To this end        geomechanical simulators employ a meshed representation as        described above and solve the equation for the conservation of        the quantity of movement in a discrete manner using the finite        element method. This technique notably makes it possible to        predict the stress variations in three dimensions in space.

The U.S. Pat. No. 8,271,243 B2 describes a method based on cooperationbetween a basin simulation and a geomechanical simulation. In theapproach described in the U.S. Pat. No. 8,271,243 B2 thethree-dimensional geomechanical stress variations to which a given layeris subjected is determined only once, only during its deposition. Inother words, the approach described in the U.S. Pat. No. 8,271,243 B2ignores the three-dimensional geomechanical effects resulting from thedeposition of subsequent layers or of tectonic movements affecting thebasin after the deposition of this layer.

This way of taking into account the evolution of the three-dimensionalgeomechanical stresses during the creation of a sedimentary basin istherefore unsatisfactory if that basin is characterized by deposits thatare variable laterally (inducing horizontal stress variations, not justvertical ones) and is subjected to tectonic stresses (generallyincluding a non-zero horizontal component). A notable possible result ofthis is a deviation in the final estimation of the pore pressures andthe fluid saturations in each of the meshes of the meshed representationrepresentative of the present time and obtained after using the methodaccording to the U.S. Pat. No. 8,271,243 B2. Now, an accurate knowledgeof this information is crucial for the hydrocarbon evaluation of asedimentary basin (for example the decision to exploit a givensedimentary basin is notably based on the knowledge of the hydrocarbonsaturation of that basin) and its exploitation (for example the decisionto use such or such a petroleum exploitation scheme is based on theknowledge of the pore pressures in the basin).

The subject matter of the present invention concerns the taking intoaccount in a basin simulation of the three-dimensional geomechanicaleffects to which a layer is subjected during its deposition and also ofthe three-dimensional geomechanical effects generated by the depositionor the erosion of that layer on the underlying layers already deposited.Moreover, the invention can make it possible to take tectonic stressesinto account. Thus the present invention aims at improved prediction ofthe quantities predicted by a basin simulation, notably the porepressures and the fluid saturations.

The Method According to the Invention

The present invention therefore concerns a method for exploiting asedimentary basin containing hydrocarbons, comprising a reconstitutionof the formation of said basin, said formation comprising at least thedeposition of two sedimentary layers, said reconstitution being carriedout by means of a basin simulator and a geomechanical simulatorcooperating with one another, by means of measurements of propertiesrelating to said basin and meshed representations representative of saidbasin at the time of the deposition of each of said sedimentary layers.On the basis of said measurements and a reconstitution of said formationof the first of said layers, said reconstitution for at least one ofsaid layers underlying said first layer is implemented according to atleast the following steps:

-   A. by means of said basin simulator, there is executed a conjoint    basin simulation of said layer and of at least one layer underlying    said layer and a first set of parameters is determined;-   B. on the basis of at least a part of said first set of said    parameters and said geomechanical simulator there is carried out a    conjoint geomechanical simulation of said layer and of at least one    layer underlying said layer, and a second set of parameters is    determined;-   C. a deviation is measured between at least a part of said    parameters of said first set and at least a part of said parameters    of said second set and the steps A) to C) are executed iteratively    applying a correction to said basin simulation if said deviation is    above a predefined threshold.

Then, by means of said reconstitution carried out for said layers, thereis selected at least one zone of said basin including said hydrocarbonsand said basin is exploited as a function of said selected zone.

According to one embodiment of the invention, said reconstitutionrelating to said first layer may be carried out according to at leastthe following steps:

-   i. by means of said basin simulator there is carried out a basin    simulation relating to said first layer and a first set of    parameters is determined;-   ii. on the basis of at least a part of said first set of parameters    and said geomechanical simulator, there is carried out a simulation    relating to said first layer and a second set of parameters is    determined;-   iii. a deviation is measured between at least a part of said    parameters of said first set and at least a part of said parameters    of said second set and the steps i) to iii) are executed iteratively    applying a correction to said basin simulation if said deviation is    above a predefined threshold.

According to one embodiment of the invention, said first set ofparameters may include at least the porosity and the pressure in each ofthe meshes of said meshed representation.

According to one embodiment of the invention, said second set ofparameters may include at least the porosity in each of the meshes ofsaid meshed representation.

Said deviation may preferably be based on a measurement of thedifference between said porosity from said first set and said porosityfrom said second set.

Said deviation may advantageously be an absolute deviation MES^(abs)defined according to a formula of the following type:

MES ^(abs)=max_(nεN)(|φ_(n) ^(g)−φ_(n) ^(b)|)

where φ_(n) ^(b) (respectively φ_(n) ^(g)) corresponds to said porositydetermined by said basin simulator (respectively by said geomechanicalsimulator) in a mesh n of said meshed representation comprising Nmeshes.

According to one embodiment of the invention, said deviation may be arelative deviation MES^(rel) defined according to a formula of thefollowing type:

${MES}^{rel} = \frac{2 \times {\max_{n \in N}\left( {{\phi_{n,}^{g} - \phi_{n,}^{b}}} \right)}}{\phi_{n}^{g} + \phi_{n}^{b}}$

where φ_(n) ^(b) (respectively φ_(n) ^(g)) corresponds to said porositydetermined by said basin simulator (respectively by said geomechanicalsimulator) in a mesh n of said meshed representation comprising Nmeshes.

According to one embodiment of the invention, said method may include astep of simulating the erosion of at least one of said layers and/or ofsimulating a geological disconformity.

The invention further concerns a computer program product that can bedownloaded from a communication network and/or stored on acomputer-readable medium and/or executed by a processor, comprisingprogram code instructions for executing the method according to theabove description when said program is executed on a computer.

Other features and advantages of the method according to the inventionwill become apparent on reading the following description of nonlimitingembodiments with reference to the appended figures describedhereinafter.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic representation of the subsoil of a petroleumbasin.

FIG. 2 shows one example of a sedimentary basin (on the left) and oneexample of a meshed representation of that basin (on the right).

FIG. 3 shows the structural reconstruction of a sedimentary basinrepresented by three deformation states at three different time periods.

FIG. 4 shows a curve representing the evolution of the effect of stressas a function of porosity.

FIG. 5 compares the prediction of the evolution of the pore pressure ina sedimentary basin obtained by the method according to the inventionand that obtained by a prior art method.

DETAILED DESCRIPTION OF THE METHOD

The invention concerns a method for exploitation of a sedimentary basincontaining hydrocarbons, notably the identification of at least one zoneof said basin in which hydrocarbons have been able to accumulate, with aview to extraction of those hydrocarbons.

One object of the invention concerns a realistic reconstitution of theformation of a sedimentary basin resulting from the deposition of atleast two sedimentary layers, said reconstitution of at least the secondlayer being produced via a basin simulator and a geomechanical simulatorcooperating with one another. Hereinafter it is considered that thedeposition of a sedimentary layer corresponds to a time period (or timestep) of a simulation. Note that a time period of a simulation maynevertheless also correspond to the erosion of a sedimentary layer or toa geological disconformity. According to the invention, based on thereconstitution of the formation of a first layer (for example the layerdeposited first in the basin), there is carried out a reconstitution fora layer underlying said first layer by applying at least one basinsimulation and one geomechanical simulation for said underlying layerconjointly with at least one underlying layer (i.e. a layer alreadydeposited, such as the first layer, for example) of that basin.

The present invention requires:

-   -   measurements of properties relating to the basin: these are        measurements carried out in situ (for example by core sampling,        logging in wells, seismic acquisition campaigns, etc.), at        different points in the basin being studied, necessary for a        basin simulation and a geomechanical simulation, such as the        porosity, permeability or lithology at the present time.        According to one embodiment of the invention aiming to take        account of the three-dimensional stresses induced by tectonic        movements, estimates of the values of the stresses in situ and        their orientation in space are required. These estimates may be        obtained from leak-off tests, for example, mini-frac (see for        example Zoback, 2010) and analysis of the ovalization of the        wells. Another possible source of information is the geological        analysis of the present geometry of the basin, which makes it        possible to estimate any tectonic extension and shortenings,        ideally to arrive at a pertinent kinematic scenario.    -   a basin simulator, i.e. software making it possible to effect a        numerical basin simulation using a computer. To be more precise,        a basin simulator makes it possible to simulate numerically the        evolution (which includes the genesis and the migration) of the        fluids (hydrocarbons but also the water of formation) in the        basin being studied and their properties (evolution of the fluid        pressures, saturations and temperatures), together with the        evolution of the petrophysical properties of the rocks        constituting the sedimentary layers of the basin being studied        (notably the porosity and the permeability). According to the        invention, a basin simulation is carried out using the basin        simulator for a succession of time periods (also termed time        steps), each period corresponding to a geological event, such as        the deposition or the erosion of a sedimentary layer, or a        geological disconformity. According to the invention, the basin        simulator requires meshed representations of the basin for each        time step of the simulation (the construction of such meshed        representations will be described during the description given        hereinafter of the steps 1 and 2). The basin simulation        therefore consists in solving a system of differential equations        describing the evolution over time of the physical magnitudes        being studied. To this end, discretization by the finite volumes        method may be used, for example, as described in (Scheichl et        al., 2003), for example. In accordance with the principles of        the finite volumes methods centered on the meshes, the unknowns        are discretized by a constant value per mesh and the equations        of conservation (of mass or heat) are integrated in space over        each mesh and in time between two successive time steps. The        discrete equations then express the fact that the quantity        conserved in a mesh at a given time step is equal to the        quantity contained in the mesh at the preceding time step plus        the quantity flows entering the mesh and minus the quantity        flows leaving the mesh via its faces, also external inputs. In        each time step and in each mesh of the meshed representation of        the basin for the time step concerned, the basin simulator        required for implementation of the invention makes it possible        at least to calculate the following physical quantities: pore        pressure and porosity. Basin simulation preferably also        determines the temperatures and the saturations. One example of        a basin simulator of this type is the TemisFlow™ software from        IFP Énergies nouvelles, France.    -   a geomechanical simulator, i.e. a program making it possible to        effect a numerical geomechanical simulation using a computer. To        be more precise, a geomechanical simulator makes it possible to        simulate the evolution of the stresses and deformations in a        sedimentary basin as it is formed. The geomechanical simulation        necessitates a meshed representation of the basin being studied        for each period (or time step) of the history of the basin for        which it is wished to produce an estimate of the geomechanical        effect. According to one embodiment of the present invention,        the geomechanical simulation is based on a discretization and a        solution of the equation for the conservation of the quantity of        movement by the finite elements method (see for example        Zienkiewicz et al., 2013). In this case, and if the meshed        representation for the basin simulation is suitable for solution        by the finite volumes method, a meshed representation suitable        for the geomechanical simulation can be determined from a meshed        representation suitable for the basin simulation by a remeshing        process. The remeshing step may consist in cutting any        degenerate hexahedra that may be present in the meshing suitable        for solution by the finite volumes method into tetrahedra        suitable for a solution by the finite elements method. In the        case of meshed representations differing between basin        simulation and geomechanical simulation, the correspondence        between the two meshed representations may also be arrived at        via a so-called mapping algorithm. The geomechanical simulator        according to the invention requires at least one pore pressure        value for each time step and in each mesh of the meshed        representation relating to the time step concerned and makes it        possible to calculate at least the tensor of the stresses and        the tensor of the deformations in each of the meshes of the        meshed representation from an initial state defined in terms of        stresses, pressures and porosities. Moreover, each mesh of the        meshed representation associated with a given time step is        associated at least with a geomechanical behavior law. The        geomechanical behavior law may be expressed by a Young's        modulus, a Poisson coefficient, an elastic limit and/or a        hardening law, which parameters can be estimated on the basis of        the facies (clay, sand, etc.) present in the mesh concerned.        According to one embodiment of the invention, the geomechanical        simulator may additionally take account of the boundary        conditions, notably making it possible to take into account        regional tectonic movements. One example of a geomechanical        simulator of this kind is the ABAQUS™ software from Dassault        Systèmes, France.

The present invention then includes at least the following steps:

1. Construction of meshed representations of the basin

2. Reconstitution of the formation of the basin

-   -   2.1. Application of a basin simulation    -   2.2. Application of a geomechanical simulation    -   2.3. Consistency check

3. Exploitation of the sedimentary basin

The principal steps of the present invention are described in detailbelow.

1. Construction of Meshed Representations of the Basin

The invention is based on a basin simulation cooperating with ageomechanical simulation.

Consequently, the method according to the invention requires a meshedrepresentation for each simulation time step concerned for theimplementation of the invention, a time step corresponding to thedeposition of a sedimentary layer, a geological disconformity or theerosion of a sedimentary layer.

According to one embodiment of the invention, this step is carried outfirstly by establishing a meshed representation of the basin beingstudied at the present time (step 1.1) and then reconstructing the pastarchitectures of the basin (step 1.2), working back from the presenttime to a geological time t before the present time. This particularembodiment of the invention is described in more detail below.

1.1 Construction of a Meshed Representation at the Present Time

A meshed representation is a maquette of the sedimentary basin generallyrepresented on a computer in the form of a mesh or grid, each mesh beingcharacterized by one or more petrophysical properties relating to thebasin, such as porosity, facies, permeability, etc. The construction ofthis model is based on data acquired from seismic campaigns,measurements in wells, core samples, etc.

To be more precise, the construction of a meshed representation of abasin consists in discretizing the architecture of the basin in threedimensions, assigning properties to each of the meshes of that meshedrepresentation and adding boundary conditions of that representation totake account of the interaction of the model area with its environment.To this end, there are notably used the measurements of propertiescarried out at various points of the basin as described above, which areextrapolated and/or interpolated in the various meshes of the meshedrepresentation according to more or less restrictive hypotheses.

The spatial discretization of a sedimentary basin is most oftenorganized in layers of meshes each representing the various geologicallayers of the basin being studied. FIG. 2 shows on the left an exampleof a sedimentary basin and on the right an example of a meshedrepresentation of that basin.

The meshed representation being usable for implementing the methodnotably comprises in each mesh information on the lithology, a porosityvalue, a permeability value and properties relating to the fluids(notably the saturation). Note that the specialist is in a position todeduce from these quantities information relating to the compaction ineach of the meshes of the meshed representation.

1.2 Structural Reconstruction

During this substep, it is a question of reconstructing the pastarchitectures of the basin, working back from the present time to ageological time t before the present time. To this end, the meshedrepresentation constructed in the preceding step is deformed in order torepresent the anti-chronological evolution of the architecture of thesubsoil during geological time periods and for each time step of thesimulation. This produces a meshed representation for each time step ofthe simulation from the present time to the geological time t.

According to one embodiment of the present invention, the structuralreconstruction may be particularly simple if it is based on thehypothesis that its deformation is the result only of a combination ofvertical movements by compaction of the sediment or by upheaval ordeflection of its base. This technique, known as backstripping, isdescribed in (Steckler and Watts, 1978), for example.

According to another embodiment of the present invention, in the case ofbasins with a complex tectonic history, notably basins including faults,it is necessary to use techniques with less restrictive hypotheses, suchas structural restoration. One such structural restoration is describedin the document FR 2 930 350 A (US 2009/0265152 A), for example. Thestructural restoration consists in calculating the successivedeformations of the basin, integrating the deformations caused bycompaction and those that result from tectonic forces. In the FIG. 3example, three states are used to represent the deformation of thesubsoil during geological time periods. The meshed representation on theleft represents the present state, in which a slippage interface can beseen (here a fault). The meshed representation on the right representsthe same sedimentary basin at a geological time t before the presenttime. At this time t the sedimentary layers were not yet fractured. Thecentral meshed representation is an intermediate state, i.e. representsthe sedimentary basin at a time t′ between the time t and the presenttime. It is seen that slippage has begun to modify the architecture ofthe basin.

2. Reconstitution of the Formation of the Basin

During this step, it is a question of reconstituting the formation ofthe basin being studied from the meshed representations established inthe preceding steps by means of a basin simulator and a geomechanicalsimulator cooperating with one another.

To do this, it is necessary to have available beforehand areconstitution of the formation of the first of the layers of thesedimentary basin being studied, i.e. the one that was deposited in thebasin first, or the oldest layer. This reconstitution of the first ofthe layers may have been obtained by any means. According to oneembodiment of the present invention, this reconstitution is obtained bya basin simulation.

Then, for at least one layer that was deposited after the first layer(i.e. a layer situated on top of the first of the layers), at least onebasin simulation is applied followed by a geomechanical simulation, saidsimulations taking conjointly into account the layer concerned and atleast one layer underlying the layer concerned, the cooperation beingassured by the transfer of some of the parameters to the geomechanicalsimulation on exiting the basin simulation.

For a given layer, the basin simulation is preferably appliedcooperatively with the geomechanical simulation for the layer concerned,conjointly with all the layers previously deposited (i.e. all the layersunderlying the layer concerned).

According to the invention, the following substeps are thereforeimplemented for each layer concerned, the substeps 2.1 to 2.3 beingrepeated for at least each layer concerned:

2.1. Application of a Basin Simulation

During this substep, it is a question of applying a basin simulation bymeans of a basin simulator to the layer concerned, conjointly with atleast one underlying layer, i.e. a layer already deposited. The conjointbasin simulation of a plurality of sedimentary layers makes it possibleto take account of the stresses imposed by the deposition of theoverlying layers on the underlying layers.

The basin simulation applied according to the invention therefore makesit possible to calculate parameters for the layer concerned and also toupdate parameters calculated for the layer previously deposited andconsidered conjointly with the current layer.

According to the invention, the basin simulation for the layer concernedis implemented using the meshed representation determined during thepreceding step 2 for the time period corresponding to the deposition ofthe current layer. According to one embodiment of the invention, saidmeshed representation that is required contains in each mesh the usualbasin simulation information, such as petrophysical properties relatingto the lithology, information characterizing the type of compaction,properties relating to the fluids present in the formation at the timeperiod concerned, as well as the limits and geometries of the layersalready deposited.

According to one embodiment of the invention, the basin simulator usedto implement the present invention makes it possible to discretize andsolve the equations described in the document (Schneider et al., 2000).The TemisFlow™ software from IFP Energies nouvelles, France is oneexample of a basin simulator of this kind.

After applying the basin simulation for the time period concerned, thereis obtained a first set of parameters such as pore pressures,saturations, temperatures, porosities in each of the meshes of themeshed representation representative of the period concerned.

According to the invention, this first set of parameters comprises atleast the porosities in each mesh of the meshed representation for theperiod concerned. According to one embodiment of the invention, thisfirst set of parameters comprises at least the pore pressures and theporosities in each mesh of the meshed representation for the periodconcerned.

2.2. Application of a Geomechanical Simulation

During this substep it is a question of producing a geomechanicalsimulation for the same time period as that considered for the precedingsubstep. According to the invention, the geomechanical simulation isconjointly applied to the layer deposited during the current period andat least one other layer deposited in an earlier time period. Theconjoint geomechanical simulation of a plurality of sedimentary layersmakes it possible to take into account the mechanical stresses imposedby the deposition of the overlying layers on the underlying layers.

The geomechanical simulation will preferably be applied to all thelayers deposited up to the time period concerned. In other words, thegeomechanical simulation at a given time period is carried out includingall the column of sedimentary layers already deposited up to thatperiod. In this way, the method according to the invention makes itpossible to simulate not only the three-dimensional geomechanicaleffects produced in the layer deposited during the time period concernedbut also the effects produced in all the layers already deposited up tothe time period concerned.

According to the invention, the geomechanical simulation is based on ameshed representation representative of the basin for the time periodconcerned. According to one embodiment of the invention, this may be themeshed representation used for the basin simulation when carrying outthe preceding substep, possibly adapted to suit the method of solvingthe equations required by the geomechanical simulation as describedabove.

According to the invention, a portion of the first set of parameterscalculated in the preceding substep for the same time period is sent tothe geomechanical simulator. According to one embodiment of theinvention, a portion of the first parameters calculated in the precedingsubstep corresponds to the pressure at the start and at the end of thetime period concerned (termed the pressure history) by the basinsimulation. This pressures history can then be used as a condition forthe geomechanical simulation relating to the time period concerned.According to another embodiment, the geomechanical simulation is alsosent the temperatures and/or saturations determined by basin simulationin the preceding substep. According to one embodiment of the invention,in the case of meshed representations differing between basin simulationand geomechanical simulation, the correspondence between the two meshedrepresentations may be established via a so-called mapping algorithm.

The geomechanical simulation preferably also takes into accountmechanical stresses linked to the regional tectonic movements to whichthe sedimentary basin being studied has been subjected. According to oneembodiment of the invention, these mechanical stresses are taken intoaccount as boundary conditions during geomechanical simulations, forexample imposing on the geomechanical simulation the verification at theedges of the simulation domain of the horizontal movements (reflecting acompression or an extension). Taking into account such boundaryconditions can be done using the finite elements method via the use ofconditions at the limits in movement or stresses. According to thisembodiment, the meshed representation for the time period concerned istherefore deformed during the geomechanical simulation step, respectingthe boundary conditions that have been established.

Following this substep, there is obtained a second set of parametersrelating to the time period concerned. According to one embodiment ofthe invention, this second set of parameters comprises at least theporosities in each mesh of the meshed representation for the periodconcerned. Note that the meshed representation at the end of a time stepof a geomechanical simulation is not necessarily identical to the meshedrepresentation from the structural reconstruction step and determinedduring step 2 for the next time step.

2.3. Consistency Check

During this step, it is a question of effecting a consistency checkbetween at least some of the parameters of the first set of parametersdetermined in the step 2.1 by basin simulation and at least some of theparameters of the second set of parameters determined in the step 2.2 bygeomechanical simulation.

To this end, according to the invention, a deviation is measured betweenat least some of the parameters of the first set (from the basinsimulation) and at least some of the parameters of the second set (fromthe geomechanical simulation) and it is verified whether that deviationis or is not beyond a certain threshold predefined by the specialist.

According to one embodiment of the invention, said deviation isevaluated by comparing the porosities estimated by basin simulation andthe porosities estimated by geomechanical simulation. The deviation canbe estimated mesh by mesh if the basin and geomechanical models haveidentical meshings; if not, a field transfer technique well known to thespecialist is used. A reference to a technique of this kind can be foundin the document (Nayroles et al., 1991) for example.

According to one embodiment of the invention, for the time step tconcerned, a deviation MES_(i) is calculated using a formula of thefollowing type:

MES _(i) ^(abs)=max_(nεN)(|φ_(n,t) ^(g)−φ_(n,t) ^(b)|)  (1)

where φ_(n,t) ^(b) (respectively φ_(n,t) ^(g)) is the porositydetermined by basin simulation (respectively by geomechanicalsimulation) into a mesh n of the meshed representation relating to thetime step t, the meshed representation comprising N meshes. Note that ifthe geomechanical simulator does not determine the porosity directly,the latter can be obtained using the formula: φ_(me)=(V_(T)−V_(S))/V_(T)where V_(T) corresponds to the total volume of the mesh and V_(s) to thesolid volume in the mesh. According to this embodiment, the porositypredictions are considered to be consistent between geomechanicalsimulation and basin simulation for the time step t concerned if thedeviation between these two predictions is below a threshold valuepredefined by the specialist. The predefined threshold value mayadvantageously be between 0.0001 and 0.02 inclusive. The porositypredictions are preferably considered to be consistent betweengeomechanical simulation and basin simulation for the time step tconsidered if MES_(t)≦0.001.

According to another embodiment of the invention, a relative deviationMES_(t) ^(rel) is calculated on the basis of a formula of the form:

${MES}_{t}^{rel} = {\frac{2 \times {\max_{n}\left( {{\phi_{n,t}^{g} - \phi_{n,t}^{b}}} \right)}}{\phi_{n,t}^{g} + \phi_{n,t}^{b}}.}$

According to this embodiment, the porosity predictions are consideredconsistent between geomechanical simulation and basin simulation for thetime step t concerned if the deviation between these two predictions isless than a value predefined by the specialist. The predefined thresholdvalue may advantageously be between 0.0001 and 0.02 inclusive. Theporosity predictions are preferably considered consistent betweengeomechanical simulation and basin simulation for the time step tconcerned if MES_(t) ^(rel)≦0.001.

According to the invention, if the consistency criterion is notverified, a correction is estimated to be applied on starting a newbasin simulation, which is executed for the same time step. The aim ofthis correction is to force the results of the geomechanical simulationand the results of the basin simulation to converge.

According to one embodiment of the invention, this correction may be acorrection of the permeabilities and/or the stresses to be applied ineach of the meshes of the meshed representation relating to the timestep concerned before the application of a new basin simulation.

According to one embodiment of the invention, a stress correction to beapplied in each of the meshes of the meshed representation at the timestep t is estimated using the difference between the porositiescalculated by the basin simulator and by the geomechanical simulation,as well as a sedimentary deposit compaction law. According to oneparticular embodiment of the invention, there is considered a compactionlaw f linking effective stresses and porosities and a stress correctionΔσ_(n,t) ^(eff) is calculated to be applied on starting the basinsimulation for a time step t and a mesh n according to a formula of thefollowing type:

Δσ_(n,t) ^(eff) =f ⁻¹(φ_(n,t) ^(g))−f ⁻¹(φ_(n,t) ^(b)),  (2)

where f represents a sedimentary deposit compaction law, which may be ofthe type:

f(σ_(n,t) ^(eff))=φ₀+φ_(a) exp(−σ_(n,t) ^(eff)/σ_(a))+φ_(b) exp(−σ_(n,t)^(eff)/σ_(b)),  (3)

where φ₀, φ_(a), φ_(b), σ_(a) and σ_(b) are properties characterizingthe compaction of the sediment concerned, which properties thespecialist can determine from measurements of properties relating tosaid basin (see step 1). FIG. 4 notably shows how to deduce from acompaction law f (continuous curve) and a porosity differential Δφ_(n,t)an effective stress differential Δσ_(n,t) ^(eff) for a mesh n at a timestep t. Taking into account stress corrections in a new basin simulationfor the current layer has the effect of influencing the evolution of theporosity during the simulated period and therefore of rapidly leading toconvergence of the parameters from the basin simulation and those fromthe geomechanical simulation.

According to another embodiment of the invention, a permeabilitycorrection is estimated to be applied in each of the meshes of themeshed representation at the time step t using estimates of the stressesobtained from the last geomechanical simulation and considerations as tothe geomechanical state of the materials deduced from their porosity orfrom their state of deterioration or of fracturing. For example, thiscorrection may lead to a significant increase in the permeability of amesh that has reached a stress state that is critical with regard to therupture criterion associated with its geomechanical behavior law. Takingpermeability corrections into account in a new basin simulation for thetime step concerned has the effect of influencing, among other things,the evolution of the pressures and overpressures, and therefore ofleading to potentially more pertinent results. According to oneembodiment of the invention, in the case of meshed representationsdiffering between basin simulation and geomechanical simulation, thecorrespondence between the two meshed representations may be assured viaa mapping algorithm.

According to another embodiment of the invention, there are estimatedboth a correction of permeabilities and a correction of stresses in eachof the meshes of the meshed representation at the time step concerned.The taking into account of stress corrections and permeabilitycorrections in a new basin simulation for the time step concerned hasthe effect of influencing the evolution among other things of theporosity, the pressures and overpressures and therefore of rapidlyleading to the convergence of the parameters from the basin simulationand from the geomechanical simulation as well as to potentially morepertinent results.

According to the invention, if the consistency of the parameters of thetwo simulations is not verified, i.e. if the deviation between at leastsome of the parameters from the basin simulation and at least some ofthe parameters from the geomechanical simulation for the current timestep is above a threshold predefined by the specialist, a correction isdetermined to be applied in the basin simulation and the substeps 2.1 to2.3 are repeated until said deviation is below said threshold.

If not, according to the invention, once the consistency of theparameters of the two simulations has been verified, i.e. if thedeviation between at least some of the parameters from the basinsimulation and at least some of the parameters from the geomechanicalsimulation for the current time step is below the threshold predefinedby the specialist, the substeps 2.1 to 2.3 described above are appliedfor the next time step.

This step therefore makes it possible to guarantee a realisticreconstruction of the basin.

3. Exploitation of the Sedimentary Basin

Following the application of the preceding step for each of the timeperiods to be considered for the reconstitution of the formation of thebasin being studied, general information is available such as:

-   i. the placement of the sedimentary layers,-   ii. their heating during their burial,-   iv. the modifications of fluid pressures resulting from that burial,-   v. the formation of the hydrocarbons formed by thermogenesis,-   vi. the movement of those hydrocarbons in the basin because of the    effect of buoyancy, capillarity, pressure gradient differences,    subterranean flows,-   vii. the quantity of hydrocarbons resulting from thermogenesis in    the meshes of each of the meshed representations of said basin,-   viii. the evolution of the three-dimensional deformations and    stresses during the formation of said basin.

On the basis of such information and notably on the basis of theinformation for the present time period, the specialist can determine atleast one area of the basin, corresponding to meshes of said meshedrepresentation of said basin at the present time, containinghydrocarbons, as well as the concentration, nature and pressure of thehydrocarbons that are trapped there. The specialist is then in aposition to select the areas of the basin being studied having thegreatest hydrocarbon potential.

The petroleum exploitation of the basin may then take a number of forms,notably:

-   -   carrying out exploratory drillings in the various areas selected        as having the highest potential in order to confirm or to        discount the potential estimated beforehand and to acquire new        data for feeding new and more precise studies,    -   carrying out exploitation drillings (producer or injector wells)        for the recovery of the hydrocarbons present in the sedimentary        basin in the area selected as having the highest potential.

Computer Program Product

The invention further concerns a computer program product that can bedownloaded from a communication network and/or stored on acomputer-readable medium and/or executed by a processor, comprisingprogram code instructions for executing the method as described abovewhen said program is executed on a computer.

Variants

According to one embodiment of the present invention the reconstitutionrelating to the first layer of the sedimentary basin concerned iscarried out according to a basin simulation cooperating with ageomechanical simulation. To be more precise, a basin simulation isperformed relating to the first layer and a first set of parameters isdetermined. Then, on the basis of at least some of the parameters of thefirst set and a geomechanical simulator a geomechanical simulation isperformed relating to the first layer and a second set of parameters isdetermined. A deviation is then measured between at least some of theparameters of the first set and at least some of the parameters of thesecond set (for example as described above in relation to the step 2.3).If this deviation is above a threshold predefined by the specialist, acorrection is calculated (for example a correction of stresses or ofpermeability, as described above in relation to the step 2.3) to beapplied in a new basin simulation for the same first layer, followed bya geomechanical simulation, and this process continued until themeasured deviation respects the threshold predefined by the specialist.

According to one embodiment of the invention, the basin simulator andthe geomechanical simulator used to carry out the steps 2.1 and 2.2described above make it possible where appropriate to simulate theerosion of at least one of the layers of the sedimentary basin concernedand/or to simulate a geological disconformity at a time periodconcerned. Said simulations of an erosion and/or a disconformity arepreferably carried out conjointly with at least one layer previouslydeposited and where applicable not affected by said erosion. Saidsimulations of said erosion and/or a disconformity are preferablycarried out conjointly on all the layers previously deposited and whereapplicable not affected by said erosion. According to this lastembodiment of the invention, the method according to the invention canmake it possible to simulate the evolution of the porosity,permeability, stress field and pressure field induced in the sedimentarybasin being studied by either total or partial erosion of a sedimentarylayer of the basin being studied and/or induced by a geologicaldisconformity.

Application Example

The features and advantages of the method according to the inventionwill become more clearly apparent on reading the following applicationexample.

The method according to the invention was employed in the case of theformation of a sedimentary basin resulting from the deposition atvarying speed of a series of layers containing clay over a period of 220million years, the clay column being almost 6000 meters deep at thepresent time. In addition to the mechanical effects linked to thesuccessive sedimentary deposits, this basin was subjected between 78 and72 million years ago to an extensive tectonic phase (10% extension),generating three-dimensional mechanical stress variations.

FIG. 5 shows two curves:

-   -   a first curve, plotted with squares, that corresponds to the        evolution over geological time T of the pore pressure P        predicted at the level of the first layer containing clay (i.e.        the deepest layer) by the method of the invention, the latter        taking into account the three-dimensional stress variations        induced by the deposits later than the layer concerned and the        stress variations stemming from tectonic movements. Moreover,        the consistency check between the set of parameters from the        basin simulation and the set of parameters from the        geomechanical simulation described above in relation to the step        2.3 was carried out with a threshold value of 0.001;    -   a second curve, plotted with triangles, that also corresponds to        the evolution over geological time T of the predicted pore        pressure P at the level of the first layer containing clay but        by a prior art method, i.e. a method taking account neither of        the three-dimensional stress variations induced by the deposits        after the layer concerned nor the stress variations resulting        from tectonic movements that occurred after the deposition of        the layer concerned.

It is seen in this figure that taking account of the three-dimensionalmechanical stresses for the deepest layer throughout the formation ofthe basin makes it possible to predict a pressure drop of 100 bar insaid layer, that pressure drop being induced by the extensive tectonicepisode (delimited on the geological time axis by a shaded area)occurring after it was deposited. This tectonic phenomenon havingoccurred after the layer concerned was deposited, it is ignored by theprior art method, which leads to a prediction error in respect of thepore pressure of 100 bar in the layer concerned at the present time. Anerror of this kind can greatly falsify the assessment of the hydrocarbonpotential of said layer. For example, the specialist might judge thepressures in the layer concerned to be insufficient to enablecost-effective hydrocarbon extraction.

Taking into account of the three-dimensional variations of themechanical stresses to which a sedimentary basin is subjected throughoutits formation therefore appears to be of fundamental importance for atrue assessment of the hydrocarbon potential of a sedimentary basin.

1. A method for exploiting a sedimentary basin containing hydrocarbons,comprising a reconstitution of the formation of said basin, saidformation comprising at least the deposition of two sedimentary layers,said reconstitution being carried out by means of a basin simulator anda geomechanical simulator cooperating with one another, by means ofmeasurements of properties relating to said basin and meshedrepresentations representative of said basin at the time of thedeposition of each of said sedimentary layers, characterized in that, onthe basis of said measurements and a reconstitution of said formation ofthe first of said layers, said reconstitution for at least one of saidlayers underlying said first layer is implemented according to at leastthe following steps: A. by means of said basin simulator, there isexecuted a conjoint basin simulation of said layer and of at least onelayer underlying said layer and a first set of parameters is determined;B. on the basis of at least a part of said first set of said parametersand said geomechanical simulator there is carried out a conjointgeomechanical simulation of said layer and of at least one layerunderlying said layer, and a second set of parameters is determined; C.a deviation is measured between at least a part of said parameters ofsaid first set and at least a part of said parameters of said second setand the steps A) to C) are executed iteratively applying a correction tosaid basin simulation if said deviation is above a predefined threshold;and in that, by means of said reconstitution carried out for saidlayers, there is selected at least one zone of said basin including saidhydrocarbons and said basin is exploited as a function of said selectedzone.
 2. A method as claimed in claim 1, in which said reconstitutionrelating to said first layer is carried out according to at least thefollowing steps: i. by means of said basin simulator there is carriedout a basin simulation relating to said first layer and a first set ofparameters is determined; ii. on the basis of at least a part of saidfirst set of parameters and said geomechanical simulator, there iscarried out a simulation relating to said first layer and a second setof parameters is determined; iii. a deviation is measured between atleast a part of said parameters of said first set and at least a part ofsaid parameters of said second set and the steps i) to iii) are executediteratively applying a correction to said basin simulation if saiddeviation is above a predefined threshold.
 3. A method as claimed inclaim 1 in which said first set of parameters includes at least theporosity and the pressure in each of the meshes of said meshedrepresentation.
 4. A method as claimed in claim 3 in which said secondset of parameters includes at least the porosity in each of the meshesof said meshed representation.
 5. A method as claimed in claim 3 inwhich said deviation is based on a measurement of the difference betweensaid porosity from said first set and said porosity from said secondset.
 6. A method as claimed in claim 5 in which said deviation is anabsolute deviation MES^(abs) defined according to a formula of the type:MES ^(abs)=max_(nεN)(|φ_(n) ^(g)−φ_(n) ^(b)|) where φ_(n) ^(b)respectively φ_(n) ^(g)) corresponds to said porosity determined by saidbasin simulator (respectively by said geomechanical simulator) in a meshn of said meshed representation comprising N meshes.
 7. A method asclaimed in claim 5 in which said deviation is a relative deviationMES^(rel) defined according to a formula of the type:${MES}^{rel} = \frac{2 \times {\max_{n \in N}\left( {{\phi_{n,}^{g} - \phi_{n,}^{b}}} \right)}}{\phi_{n}^{g} + \phi_{n}^{b}}$where φ_(n) ^(b) (respectively φ_(n) ^(g)) corresponds to said porositydetermined by said basin simulator (respectively by said geomechanicalsimulator) in a mesh n of said meshed representation comprising Nmeshes.
 8. A method according to claim 1 in which said method includes astep of simulating the erosion of at least one of said layers and/or ofsimulating a geological disconformity.
 9. A computer program productthat can be downloaded from a communication network and/or stored on acomputer-readable medium and/or executed by a processor, comprisingprogram code instructions for executing the method as claimed in claim 1when said program is executed on a computer.